Sent folder copy of MDN Request messages should have the
MSG_FLAG_MDN_REPORT_NEEDED flag turned off otherwise if the user mark it unread
and then read it again will trigger the MDN report generation process.

Guess what, 4.x has the same problem. Oops, I missed to test that on 4.x.....
Imap doesn't have this problem only Pop3. I tried to turned off the mdn needed
flag and turned on the mdn sent flag. Althoughm the flags get written to the
X-Mozilla-Status2: header the problem still exists. Not putting the request for
return receipt header to the fcc message may be the way to go after.

Does this mean that the user will send out another request for a return receipt
to the recipient or that the user will potentially reply to their own receipt or
that the user will reply again to the sender?

If the user opens their sent folder, and reads the sent message, they will
potentially be prompted to send out another return receipt (or send it
automatically if that's their setting). It's also possible that the filtered
messages MDN bug might come into play if it isn't fixed, so that just opening
the sent folder might cause a prompt to send the return receipt.

using commercial builds:
2002-04-16-06-trunk NT 4.0
2002-04-16-07-trunk linux 2.2
2002-04-16-03-trunk mac 10.1.3
Ok assuming assuming I understand this bug: this bug fixes
any weird cases where the mesg that has RR on it,
if marked new again, and read will reprompt the user or
auto send a RR.
This is what I tested:
-Sent a mesg to myself with RR on, read it, sent RR back,
marked old mesg as new (via green dot) and reread it again. No RR prompt
occurred. Both IMAP/POP acnts.
-Tested POP accounts where I went into sent folder (under Pop acnts
since can't test sent folder under local folders due to bug 135987)
marked mesg as new and read it. NO RR prompt or auto send of RR.
I'm confused about David's comment 4 for Imap. I wasn't sure
the fix was intended just for Pop or Imap or both accounts.
I think it was intended for both accounts. If so, then I found
a problem with Imap accounts.
Steps to reproduce:
1.Create a imap mail account.
2.Turn Return Receipts on both (request for RR and ask you
if you are prompted for RR)
3.Make sure your Sent folder is located under your Imap account
4.Compose a mesg to yourself and send it w/RR on.
5.Get the mesg
6.Click on the mesg and RR prompt appears
7.Send the RR back
8.Get the mesg
9.RR is in your inbox (assuming that is where you wanted RR to be)
10.Click on your sent folder
11.Scroll down to copy of the orig mesg in step 4
12.Mark it as new via clicking on green dot.
13.Mesg should be bold
14.Click on the mesg
result: Return Receipt prompt appears
Expected: Return Receipt to not appear
If try to mark it as new again and all subsequent tries afterwards,
the RR prompt never appears. It only appears the very FIRST TIME
you try this.
reopening. If I'm under the wrong assumption please let me know.

The fix is really meant for pop3 accounts only. Imap accounts used to work okay.
Could we change the title of this bug for Pop3 and create a new bug to address
the Imap problem. The bug is related to the way we download message headers from
imap server. It also involves with custom headers filter bug.

Using commercial branch
2002-04-23-08-1.0.0 nt 4.0, linux 2.2
2002-04-23-05-1.0.0/ mac 9.2.2
2002-04-23-05-1.0.0/ mac 10.1.3
verified that any copy of mesg that's in the sent folder
(under the acccount only) of a pop account, if you mark it
as new and click on it, you will not be prompted to send Return
Receipt.
Verified on pop accounts only.
removing keyword fixed1.0.0 and replacing it with verified1.0.0
Verified on branch.